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    <title>Nim by Example - Seqs</title>

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<article>
    <h1 id="seqs">Seqs</h1>

    <p>Seqs, abbreviated from “sequence”, provide dynamically expandable storage.</p>

    <p>There are two ways to create seqs, with the <code>@</code> operator and with the <code>newSeq[T](n: int)</code>
        method. Once a seq is created, it can be modified using the <code>add(item: T)</code>, <code>delete(idx:
            int)</code>. The length of a seq can be found through <code>len: int</code>, and the maximum index through
        <code>high: int</code>. The standard <code>items: T</code> and <code>pairs: tuple[i: int, v: T]</code> iterators
        are also available.</p>

    <pre><code class="language-nimrod"><span class="kd">var</span>
  <span class="n">a</span> <span class="o">=</span> <span class="o">@[</span><span class="mi">1</span><span
                class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span
                class="o">]</span>
  <span class="n">b</span> <span class="o">=</span> <span class="n">newSeq</span><span class="o">[</span><span
                class="kt">int</span><span class="o">]</span><span class="p">(</span><span class="mi">3</span><span
                class="p">)</span>

<span class="k">for</span> <span class="n">i</span><span class="p">,</span> <span class="n">v</span> <span
                class="ow">in</span> <span class="n">a</span><span class="p">:</span>
  <span class="n">b</span><span class="o">[</span><span class="n">i</span><span class="o">]</span> <span
                class="o">=</span> <span class="n">v</span><span class="o">*</span><span class="n">v</span>

<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="mf">4</span><span class="p">..</span><span
                class="mi">100</span><span class="p">:</span>
  <span class="n">b</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span
                class="n">i</span> <span class="o">*</span> <span class="n">i</span><span class="p">)</span>

<span class="n">b</span><span class="p">.</span><span class="n">delete</span><span class="p">(</span><span
                class="mi">0</span><span class="p">)</span>  <span class="c"># takes O(n) time</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">a</span><span class="o">[</span><span
                class="mi">0</span><span class="o">]</span> <span class="o">&amp;</span> <span class="n">b</span>  <span
                class="c"># Same as original b</span></code></pre>

    <h2 id="immutability">Immutability</h2>

    <p>Sequences are dynamically allocated (i.e. allocated on the heap, not the stack), but they are immutable unless
        marked as <code>var</code>. That means</p>

    <pre><code class="language-nimrod"><span class="k">let</span> <span class="n">a</span> <span
            class="o">=</span> <span class="o">@[</span><span class="mi">1</span><span class="p">,</span> <span
            class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="o">]</span>
<span class="n">a</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span
                class="mi">4</span><span class="p">)</span></code></pre>

    <p>will fail to compile because <code>a</code> cannot be assigned to. However,</p>

    <pre><code class="language-nimrod"><span class="kd">var</span> <span class="n">b</span> <span
            class="o">=</span> <span class="o">@[</span><span class="mi">1</span><span class="p">,</span> <span
            class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="o">]</span>
<span class="n">b</span><span class="p">.</span><span class="n">add</span><span class="p">(</span><span
                class="mi">4</span><span class="p">)</span></code></pre>

    <p>will work without any problems. Sequences passed as “argument by value” are not modifiable. For example, the
        following will fail to compile.</p>

    <pre><code class="language-nimrod"><span class="k">proc </span><span class="nf">doSomething</span><span
            class="p">(</span><span class="n">mySeq</span><span class="p">:</span> <span class="kt">seq</span><span
            class="o">[</span><span class="kt">int</span><span class="o">]</span><span class="p">)</span> <span
            class="o">=</span>
  <span class="n">mySeq</span><span class="o">[</span><span class="mi">0</span><span class="o">]</span> <span class="o">=</span> <span
                class="mi">2</span>  <span class="c"># this is a compile-time error</span>
<span class="kd">var</span> <span class="n">testSeq</span> <span class="o">=</span> <span class="o">@[</span><span
                class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span
                class="mi">3</span><span class="o">]</span>
<span class="n">doSomething</span><span class="p">(</span><span class="n">testSeq</span><span class="p">)</span></code></pre>

    <p><code>seq</code> arguments can be mutable if they are passed as “argument by reference”, ie. the parameter is
        annotated with the <code>var</code> or <code>ref</code>:</p>

    <pre><code class="language-nimrod"><span class="k">proc </span><span class="nf">foo</span><span
            class="p">(</span><span class="n">mySeq</span><span class="p">:</span> <span class="kd">var</span> <span
            class="kt">seq</span><span class="o">[</span><span class="kt">int</span><span class="o">]</span><span
            class="p">)</span> <span class="o">=</span>
  <span class="n">mySeq</span><span class="o">[</span><span class="mi">9</span><span class="o">]</span> <span class="o">=</span> <span
                class="mi">999</span>

<span class="kd">var</span> <span class="n">thisSeq</span> <span class="o">=</span> <span class="n">newSeq</span><span
                class="o">[</span><span class="kt">int</span><span class="o">]</span><span class="p">(</span><span
                class="mi">10</span><span class="p">)</span>
<span class="n">foo</span><span class="p">(</span><span class="n">thisSeq</span><span class="p">)</span>

<span class="n">assert</span> <span class="n">thisSeq</span><span class="o">[</span><span class="mi">9</span><span
                class="o">]</span> <span class="o">==</span> <span class="mi">999</span></code></pre>

    <p>You can copy a sequence passed as “argument by value” and modify the copy:</p>

    <pre><code class="language-nimrod"><span class="k">proc </span><span class="nf">doSomething</span><span
            class="p">(</span><span class="n">mySeq</span><span class="p">:</span> <span class="kt">seq</span><span
            class="o">[</span><span class="kt">int</span><span class="o">]</span><span class="p">)</span> <span
            class="o">=</span>
  <span class="kd">var</span> <span class="n">varMySeq</span> <span class="o">=</span> <span
                class="n">mySeq</span>  <span class="c"># copy the seq</span>
  <span class="n">varMySeq</span><span class="o">[</span><span class="mi">0</span><span class="o">]</span> <span
                class="o">=</span> <span class="mi">999</span>
  <span class="n">assert</span> <span class="n">varMySeq</span><span class="o">[</span><span class="mi">0</span><span
                class="o">]</span> <span class="o">==</span> <span class="mi">999</span>
<span class="kd">var</span> <span class="n">testSeq</span> <span class="o">=</span> <span class="o">@[</span><span
                class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span
                class="mi">3</span><span class="o">]</span>
<span class="n">doSomething</span><span class="p">(</span><span class="n">testSeq</span><span class="p">)</span>
<span class="n">assert</span> <span class="n">testSeq</span><span class="o">[</span><span class="mi">0</span><span
                class="o">]</span> <span class="o">==</span> <span class="mi">1</span></code></pre>

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